They're called bully sticks, but not for the reason people might think.

These dog treats are made from 100 percent bull penises, and a recent study at the Cummings School of Veterinary Medicine at Tufts University concluded they may contain harmful bacteria and enough calories to add pounds to your pooch.

Dr. Lisa M. Freeman, a veterinary nutritionist who was the first author of the study, said 26 bully sticks were tested for bacterial contaminants. One was contaminated with clostridium difficile; another with methicillin-resistant staphylococcus aureus, a type of staph bacterium that is resistant to certain antibiotics; and seven were contaminated with Escherichia coli, including one tetracycline-resistant sample.

“The number of treats sampled was small, and not all of these bacterial strains have been shown to infect humans,” she said.

Still, researchers advise all pet owners to wash their hands after touching such treats, as they would with any raw meat. Additionally, the very young, elderly, pregnant, immuno-compromised and others considered at high risk should avoid all contact with raw animal-product based treats and raw meat diets, the study said.

She said she believes if more bully sticks are tested, other bacteria — including salmonella — may also be found. Salmonella is found at high rates in other raw meat-type treats, such as pig and cow ears and rawhide.

“There are a number of possible concerns for pets eating these treats or people in the household handing them,” she said. “Dogs can also shed the bacteria in their feces. These are raw meat products, but people are not thinking of them in that way.”

She said people should be aware that the sticks may be contaminated and avoid them or use caution if they decide to feed them to their pets.

Another issue is the large number of calories in bully sticks, the study found.

“The reason we decided to do the study is the misconceptions about these products, including the calorie content,” Dr. Freeman said. “We heard from people who didn't think the treats had many calories or underestimated the calories. Some thought they had no calories at all.”

On average, she said research showed an average of 15 calories per inch in bully sticks.

“They add quite a lot of calories to a dog's diet,” she said. “If a 10-pound dog eats a 6-inch bully stick, that is one-third of its daily calories.”

Bully sticks and other treats are not nutritionally balanced and should make up only a small percentage of a dog's diet, she said.

Like humans, dogs and cats can get heart disease, and obesity can also increase their risk of diabetes and arthritis, she said.

“There are really important health effects from obesity,” she said. “If they are overweight, it significantly shortens their life span.”

The problem is excessive calories, Dr. Freeman said, and people need to be aware of the calorie content of whatever treats they feed their pets, including dog biscuits and table food.

“People are not always aware what they are feeding to their dog,” she said.

Not so for Susan A. Zwicker, owner of Sue's Barkery, on Elm Street in Leominster.

Ms. Zwicker said she pays close attention to what her dogs — Mitch, a 1-year-old Jack Russell mix with a “sensitive” stomach, and Lilly, a pure-bred Japanese chin with finicky tastes — are eating.

She opened the business about a month ago, she said, and has been busy ever since making dog treats fresh to order with all natural ingredients such as dehydrated sweet potatoes and pumpkins and no preservatives.

Her customers, she said, are looking for dog treats with more natural ingredients for the same reason they are looking for natural foods for themselves.

But Robert D. Gibson said his customers keep coming back for more of them.

Mr. Gibson and his wife, Melissa “Mel” L. Gibson, own Gibson Kennels in Grafton and Gibson's Natural Pet in Worcester. More than 50 percent of their customers buy bully sticks, Mr. Gibson said.

He said he read the Tufts study and believes researchers exaggerated the harmfulness of the bacteria found.

“After I saw the report, 10 minutes later I gave my dogs a bully stick,” he said. “The way I see it, some of the bacteria they did find is harmless to dogs. Do you know how much raw meat is fed to dogs on a daily basis that contains some form of bacteria?” He sells hundreds of bully sticks a week, he said, despite their odor, because dogs love them and it keeps them busy.

As for bully sticks making dogs fat, he agrees with Dr. Freeman that moderation is key when feeding pets any food or treats.

“It is a matter of people being smart,” he said. “Don't feed your dogs so much. If I go to the doughnut shop and eat four doughnuts a day, I'm going to get fat, too.”

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